The present invention relates to oxidation catalysts usable in particular for the full oxidation to CO2 and H2O of volatile organic compounds (VOC) and in the processes in which said catalysts are used.
A characteristic of the catalysts is the oxidation of VOC compounds with the selective formation of carbon dioxide only. This is an evident advantage with respect to the known types of oxidation catalyst, in which the combustion of the VOC compounds is accompanied by the formation of CO, which besides being a toxic component implies an energy loss when the combustion of the VOC compounds is used to generate energy.
The known types of oxidation catalyst used for the combustion of VOC compounds are essentially of two types:
a) catalysts based on noble metals: they are characterized by high activity even at relatively low temperatures (250-450xc2x0 C.), but their cost is very high and is rising considerably owing to the scarcity of the metals and to their increasing demand, entailing problems in using them for applications such as the combustion of VOC compounds;
b) catalysts based on mixed oxides, such as copper chromites and barium hexaluminates, which are far less active than catalysts containing noble metals and require very drastic operating conditions; and catalysts based on rare earth complex oxides, alkaline-earth metals and transition metals (disclosed in U.S. Pat. No. 5,242,881) or having the formula La (1-x) Srx CrO3, the latter being also used to treat the emissions of internal-combustion engines (U.S. Pat. No. 5,286,698) or having the formula Ba2Cu3O6, which are selective in the oxidation of VOC compounds toward the formation of carbon dioxide, but are highly reactive toward CO2 and therefore tend to passivate irreversibly.
The catalysts according to the present invention comprise mixed oxides of Cu, Mn and rare-earth metals, in which the metals can assume multi-valence states, having a composition by weight expressed as the oxides that are specified hereafter: 10 to 75% as MnO, 8 to 50% as CuO, and 2 to 15% as La2O3 and/or as oxides of the other rare-earth metals at the lowest valence state.
Preferably, the composition is 50-60% MnO, 35-40% CuO, 10-12% La2O3.
The mixed oxides that form the active components of the catalyst have the characteristic of being p-type semiconductors (in these semiconductors, conductivity increases exponentially with the temperature according to an Arrhenius-type law and the charge vectors are electron vacancies). In these oxides, the gaseous oxygen is chemisorbed onto the surface and participates in the oxidation reaction together with the lattice oxygen.
The oxides are supported on porous inorganic carriers such as alumina, silica, silica-alumina, titanium dioxide, magnesium oxide. Gamma alumina, in the form of microspheroidal particles with an average diameter of 30-80 microns, is the preferred carrier for using the catalysts in fluid-bed reactions. For fixed-bed reactions, preference is given to the use of carriers having a definite geometric shape, such as three-lobed cylindrical granules with mutually equidistant through bores at the lobes. The dimensions of the granules are generally from 3 to 10 mm in height, the diameter of their circumscribed circumference is 5 to 10 mm, and the ratio between the geometric area and the volume of the solid part of the granule is greater than 1.3 mmxe2x88x921. The oxides are supported in an amount of generally 5 to 60% by weight, preferably 20-30% by weight.
The catalyst in tablets is prepared by impregnating the carrier initially with a solution of a salt of lanthanum or cerium or of another rare-earth metal, drying the carrier and then calcining it at a temperature around 600xc2x0 C. The carrier is then impregnated with a solution of a salt of copper and manganese, subsequently drying at 120-200xc2x0 C. and calcining up to 450xc2x0 C.
Any soluble salt can be used.
Examples of salts are nitrates, formates and acetates. Lanthanum is used preferably as lanthanum nitrate La(NO3)3; copper and manganese are preferably used as nitrates, respectively Cu(NO3)2 and Mn(NO3)3. The preferred impregnation method is dry impregnation, using an amount of solution equal to, or smaller than, the volume of the pores of the carrier.
As already noted, the catalysts selectively oxidize the VOC compounds to carbon dioxide: this occurs even when working for a limited time with an oxygen deficit with respect to the stoichiometric value required by the oxidation reaction.
With respect to catalysts based on noble metals, the catalysts according to the invention are characterized by greater resistance to sintering.
For example, after treatment at 1000xc2x0 C. in dry air, while the complete conversion temperature rises slightly for the catalysts according to the invention, it rises considerably for catalysts based on noble metals, owing to the remelting of the surface area caused by sintering of the metal particles that are present on the carrier. The catalysts are preferably used in the treatment of gaseous effluents from plants such as plants for the production of organic compounds, tire manufacture, asphalt blowing, wastewater treatment, and offset printing. The catalysts can also be used in the oxidation of NO and NO2. Another application of particular interest is the purification of gases from reactors for solid-state polycondensation of aromatic polyester resins (the impurities are mainly constituted by ethylene glycol), in which the catalysts are capable of completely oxidizing the impurities, with exclusive formation of carbon dioxide even when using the stoichiometric quantity of oxygen relative to the methane equivalents of the impurities that are present. In tests conducted by continuously feeding a nitrogen stream containing 1600 ppm of ethylene glycol on a fixed bed of the catalyst having the composition given in example 1, it was found that the ethylene glycol is removed quantitatively by using the stoichiometric amount of oxygen (5/2 moles per mole of glycol) working at 310xc2x0 C. and with a space velocity of 10000 hxe2x88x921. Selectivity to CO2 is complete.
Another application of the catalysts is the catalytic combustion of methane in thermal power stations for generating electricity. In this application, the catalysts have the advantage, with respect to combustion with a catalyst of a known type, that they can operate at lower temperatures, at which NO is not generated: this allows to avoid the post-treatments for removal of this oxide that are instead required with known types of catalyst.
The following examples are provided to illustrate but not to limit the scope of the invention.